Supercharged Chevy Small Block - Boosted Bumpsticks

We Compare Cams On A Blown & N/A Small-Block.

Choosing the right cam for your supercharged small-block makes all the difference in the world.

One of the common misconceptions about cam profiles for forced induction applications is that you have to do everything you can to minimize the dreaded overlap. In fact, some experts go so far as to say you should run stock cam profiles in blower applications.

Real-world dyno testing, of course, indicates otherwise as blower motors respond well to a variety of cam profiles that were originally designed for normally aspirated applications. The argument in favor of the stock cam is that the short duration and wide lobe separation angle eliminates overlap. Overlap is thought to be the enemy of boosted applications, as (so goes the thinking) that boost is allowed to escape out the exhaust.

In reality, cam duration beyond stock specifications is used in a wide variety of high-performance blower applications to determine the effective engine speed. Just as with a normally aspirated motor, the cam specs (primarily duration) can be used (along with intake runner length) to determine where the motor will produce peak power and torque. Shifting the torque curve higher in the rev range will usually produce a higher peak power level, but usually low-speed power will suffer in the trade off. You might suspect that this trade off has a negative effect on power once you add boost, but our testing on this supercharged small-block indicated otherwise.

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You'll be able to read about the build-up of this low-buck long-block in a future issue.

The idea behind this test was to dispel the notion that mild cam timing is best for supercharged applications. To properly test this, we ran a pair of Xtreme Energy cams in our small-block test engine. To provide complete information to the readers, we ran the motor in both normally aspirated trim and equipped with a Vortech supercharger (with both cams). This way we could illustrate the power gains offered by the change in cam timing in both normally aspirated and supercharged trim. Would the gains be greater N/A or after we added boost? Questions like these are why we spend so much time on the dyno.

Our test motor was an iron-headed, L98 350 originally plucked from the local wrecking yard. When we built it, the goal was 400 hp on the cheap so we added a set of Pro Comp aluminum cylinder heads, a Comp Xtreme Energy cam and single-plane induction system, all for around $1,100. Since we had it at our disposal, we decided to utilize it in this supercharged cam shootout. The side benefit to this motor was that the larger of the two cams we intended to test was already installed.

First let's take a look at the cams we intended to run in this comparison. In the mild corner was the Xtreme Energy XR258HR. Basically a step or two above the stock L98 cam, the XR258HR offered a lift split of 0.480 in/0.487 ex, a duration split at 0.050 of 206/212 degrees and a 110-degree lobe separation angle. This was a mild cam designed to offer good idle quality and plenty of low-speed torque. Obviously these specs also minimized overlap.

By contrast, the larger XR282HR cam was a real performance piece. The XR282HR featured both more lift and duration, checking in with a lift split of 0.510/0.520, a duration split (@0.050) of 230/236 degrees and the same 110-degree lobe separation angle. Now the question was which one would provide the best overall power curve. Naturally we suspected the motor would make more power with the larger cam, but would the gains offered continue once we added boost? What we were looking to find out was which one made the best blower cam.

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Choosing the right cam for your supercharged small-block makes all the difference in the world.

You'll be able to read about the build-up of this low-buck long-block in a future issue.

The untouched 195cc intake ports flowed 244 cfm, or more than enough to support well over 400 hp.

The boost for our test came courtesy of a Vortech centrifugal supercharger. The S-Trim (V1) blower offered more than enough airflow for our mild small-block.

The short-block was upgraded with a set of as-cast Pro Comp 195cc aluminum heads.

Our test centered on a pair of Comp Xtreme Energy cams (XR282HR shown). The idea was to illustrate the differences offered by the pair in both normally aspirated and supercharged configuration. The XR258HR offered a 0.480/0.487 lift split, a 206/212 duration split and a 110-degree lobe separation angle. The larger XR282Hr cam featured a 0.510/0.520 lift split, a 230/236 duration split and the same 110-degree lobe separation angle.

The impeller was an ultra-efficient curved-vein design that maximized both flow and response rate.

The Vortech kit was originally designed for a carbureted application. The kit included a sealed carburetor enclosure to fully pressurize the carburetor.

A simple 90-degree, cast-aluminum elbow was supplied with the Vortech kit to feed air from the discharge of the supercharger to the carb enclosure.

The Vortech universal carbureted kit was supplied with a 6-inch crank pulley and a 3.33-inch blower pulley (other pulleys are available). This produced a peak boost reading of 8 psi with the larger cam on our modified small-block.

The fuel rails required us to raise the carburetor enclosure using a carb spacer (under the enclosure). This change required use of a spacer between the original cast-aluminum elbow and the inlet into the enclosure. Lucky for us, the bolt pattern of the elbow flange matched that of a 5.0L Ford EGR spacer. Accufab took one of their aluminum spacers for a 90mm throttle body and machined it down to the required 1.23-inch thickness.

To facilitate ease of tuning, we elected to replace the carburetor with electronic fuel injection. The carburetor was replaced with this 4-hole throttle body from Accufab. Note the Accufab throttle body featured an idle air motor, and TPS sensor as well as a pair of vacuum/boost ports.

Installation of the blower kit for our test required knocking a hole in the factory oil pan. Vortech supplied the necessary punch, tap and fitting for the oil drain.

We relied on a set of 36-pound injectors from FAST for our injected 350. A tad small for this supercharged application, it was necessary to run over 50 psi of rail pressure to feed our 560 hp motor.

Testing was run on the Superflow 902 engine dyno using a FAST XFI management system.

Before running the supercharger, the engine was tested in normally aspirated trim. Equipped with the small XR258HR cam, the 350 produced peak numbers of 358 hp at 5,400 rpm and 396 lb-ft at 3,800 rpm.

Firing the fuel was an MSD billet distributor and Digital 7 ignition amplifier. MSD also supplied the plug wires for our application.

With the testing completed with the milder cam, off came the carb enclosure and intake assembly, Pro Comp valve covers, rockers and pushrods.

Next we dropped the oil pan down enough to facilitate removal of the front cover.

Out came the XR258HR cam and in went the larger XR282HR cam. We reused the factory roller lifters, guides and hold down.

The results of the cam comparison on the normally aspirated 350 Chevy were not terribly surprising. A comparison between the XR258HR and XR282HR cam revealed that the smaller cam offered better low-speed torque production up to 3600 rpm, but lost out to the larger cam thereafter. Looking strictly at the peak values, we see that the motor produced 396 lb-ft of torque with the smaller cam to 405 lb-ft with the larger cam. Had we not included a complete graph, you'd be tempted to think they offered similar torque curves, but nothing cold be further from the truth. The big cam not only offered more peak torque, but it carried that torque production (exceeding 400 lb-ft) from 3,700 rpm to 5,200 rpm. This explains the dramatic difference in peak power. The larger cam allowed the motor to exceed 400 hp, peaking at 407 hp at 5,800 rpm.

Adding the supercharger to the equation changed things dramatically. No longer did the smaller cam offer more low-speed power. The larger cam out-powered the smaller cam from 3,000 rpm to 6,200 rpm. Obviously the larger cam was a better match with the supercharger. To put this into perspective, the supercharged combination with the small cam only made 50 more horsepower than the normally aspirated combination with the big cam. Another telling factor is the fact that the boost pressure was high with the smaller cam. Running the same pulley combinations, the peak boost registered with the big cam was 8.1 psi but this increased to 9.0 psi once we installed the smaller cam. The boost was backing up in the intake due to the restrictive cam timing. Basically, the 350 made less power with more boost with the small cam.

Once again, the engine was run in normally aspirated trim before applying boost. Equipped with the larger cam profile, it produced 407 hp and 405 lb-ft of torque.

Adding the supercharger further increased the peak power output to 560 hp at 6,200 rpm and 501 lb-ft at 5,000 rpm.

Can Camshaft Lobe Separation Angles give you more performance. We put it to the test and check out what the results are. Only at www.chevyhiperformance.com, the official site for Chevy High Performance Magazine. » Read More